I had the EGR flow excessive code. I cleaned the egr valve and all the ports. I re-set the cel with code reader at auto zone. The cel stayed off for over 400 miles and it just came back on today.
How do I check the egr valve to see if it is working? I had the engine at operating temp and disconected the egr wire from egr valve and did not notice a difference in engine idle speed. The car runs fine and it getting the correct gas mileage.

 

YouTube - Emission System - E.G.R. Valve Testing & Replacement - 1 of 2

 

So I cleaned the EGR system, reset the CEL, 400 miles later the CEL came back on. New code is PO421, catalyst...

How come I didnt have the PO421 code when I had the EGR code?

I have and exhaust leak after the first cat. Can this cause the PO421 code?

 

check your spark plugs if there good, check your 02 sensors if there fuctioning then you need a new cat converter. i had the same code on mine we ended up changing the cat.

 

exhaust leak can cause p0421, but your precat is most likely fried. the broken cat can also cause misfires by causing high back pressure. If you replace it make sure you take off the exhaust pipe after the precat and dump all the rocks out of it. The rocks are whats left of the catalyst, and whats clogging the line at random times